The people in a society can be divided into two kind one group is moderate who except the truth and the another of those whose hearts are full of hardness and Harshness. In this matter the Holy Quran says:
What! is he whose heart Allah has opened for Islam so that he is in a light from his Lord like the hard-hearted? Nay, woe to those whose hearts are hard against the remembrance of Allah; those are in clear error.
Has not the time yet come for those who believe that their hearts should be humble for the remembrance of Allah and what has come down of the truth? And that they should not be like those who were given the Book before, but the time became prolonged to them, so their hearts hardened, and most of them are transgressors.
But on account of their breaking their covenant We cursed them and made their hearts hard; they altered the words from their places and they neglected a portion of what they were reminded of; and you shall always discover treachery in them excepting a few
Does God become the cause of someone being hard-hearted?
The answer is certainly in the negative because, in many cases, it is due to their wrong deeds that people deprive themselves of the mercy and guidance of God. In fact, their misdeeds become a fountainhead of mental deviation and demoralisation which renders them unable to save themselves from their ill effects and they are never inclined to accept the truth as the Holy Quran explains:
And We reveal of the Quran that which is a healing and a mercy to the believers, and it adds only to the perdition of the unjust.
This is because the unjust oppressors who usually oppress their own selves adopt a wrong meaning effect from the Holy Quran, that is, as a result of their denial, unbelief, oppression and hypocrisy the essence of their being turns into a totally different being and hence they get up to fight with the light of the truth where ever they find it. This fight with truth adds to the dirt and uncleanness of their hearts and strengthens their inclination toward rebellion.
However, the reason why Allah says that He caused the aforesaid misguidance or hard-heartedness or some other evils is that the effect of every cause is as per the Divine wish. This does not negate the freedom and choice of action granted to man because the concerned cause has been provided by man himself. They see what is bound to be seen in a valley wherein they desired to walk.
Signs of Hard-heartedness
In a hadith mentioning Almighty's Allah's revelation to Moosa (a.s.) we see: O Moosa ! Do not lengthen they ambitions in this world that may harden your heart. The one whose heart hardens gets away from Me.
And this hard-heartedness resulting from lengthy ambitions and consequent sins becomes even harder than stone as mentioned in the Holy Quran: Then your hearts hardened after that, so that they were like rocks, rather worse in hardness; and surely there are some rocks from which streams burst forth, and surely there are some of them which split asunder so water issues out of them, and surely there are some of them which fall down for fear of Allah, and Allah is not at all heedless of what you do.
This verse was sent down by God about Bani Israel's hard-heartedness. They had observed all the miracles from Moosa (a.s.). Their hearts were so stony that no admonition affected their hearts!